1. Field of The Invention
The present invention relates to an image reading apparatus and image reading method. More specifically, this invention concerns an image reading apparatus, for example, an image scanner having a plurality of sensor lines disposed in parallel at a specific interval or one stripe-type sensor line and the image reading method.
2. Description of The Prior Art
A known type of image reading apparatus such as an image scanner has three sensor lines disposed at a specific interval for three kinds of colorsxe2x80x94red, green and blue. The method of simultaneously reading the color data covered under the three sensor lines is called the three line simultaneous reading technique.
FIG. 2(a) is a conceptual presentation of the arrangements of the image sensor array of the three line simultaneous reading type. The sensor line to read red data is made up of a plurality of R sensor elements; the sensor line to read green data, of a plurality of G sensor elements; and the sensor line to read blue data, of a plurality of B elements.
It is understood that the main scanning direction corresponds to the direction in which the sensor elements of a kind are arranged to form a sensor line and the sub-scanning direction to the direction in which the image sensor array is moved for scanning.
Shown in FIG. 16 is a system diagram of the prior art apparatus for reading color images by scanning the document with such image sensor arrays.
To be more specific, three color data picked up by an image sensor array 100 are read by line reading means 102. The color data thus read are not uniform in output level among the sensor elements because of ununiformity in the amount of light from the light source, sensor element sensitivity and the like even when the document has an uniform shade, such as a sheet of black, white paper. This output data difference in output level among the sensor elements is corrected by shading correction means 105, and the color data read is accumulated in a color data delay buffer 1600. This color data delay buffer 1600 holds color data as they are inputted until after the other color data for one picture element are inputted. When all the three color data for one picture element or the same position on the document are ready, those data are mapped at a position on an image memory 110 by same picture element data mapping means 1601.
The line reading means 102 is made up of three amplifiers 103a, 103b, 103c that amplify the respective color data picked up by the image sensor array 100 and an analog to digital (A/D) converter circuit 104 for converting the analog values of those amplifiers to digital values.
The prior art image reading apparatus thus configured reads an image the following way:
First, the image sensor array 100 made up of red, green and blue sensor lines, each having sensor elements lined up, read color data simultaneously. The respective color data read are referred to shading correction means 105 through the amplifiers 103 and A/D converter circuit 104. The shading correction means 105 then subjects data of each color to shading correction. The color data thus obtained are mapped into the image memory 110 for display on CRT or the like.
Now, the mapping of color data into the image memory space will be explained.
Let it be supposed that the color data of a document at a point given by coordinates (X, Y) are to be read using an image sensor array in which red, green and blue sensor lines are disposed from the starting point in the sub-scanning direction in that order as shown in FIG. 2(a).
First, a specific B sensor element, a constituent in the sensor line for blue, reads a blue data C-B as shown in FIG. 3(a). This blue data C-B is then stored at the read color data delay buffer 1600 as shown in FIG. 16.
In the next step, as a specific G sensor element, a constituent in the sensor line for green, gets to the position given by the same coordinates (X, Y) on the document, the sensor element reads the green data C-G. This C-G is also stored at the read color data delay buffer 1600.
When a specific R sensor element, a constituent in the sensor line for red, reaches the same position given by coordinates (X, Y) on the document, the red data C-R is read in and then mapped by the same picture element mapping means 1601 into the image memory 110 at the identical coordinates along with the blue data C-B and the green data C-G that have been stored by the color data delay buffer 1600.
As set forth above, the respective color data from a specific position on the document are held in the color data delay buffer 1600 until all three color data for that position are read. Then the three color data are simultaneously brought into the same position in the image data memory space.
There are also other types of apparatuses for inputting images by the image scanner. They include:
(1) Light source switching type which uses a light source having the spectral characteristics of the primary colors for color images and one image sensor array for black and white. The light source is switched on for three primary colors one after another to separate the image data on the color document into three primary color data.
(2) Stripe image sensor array type in which a three-color stripe filter is pasted over the light intake face of the sensor unit. In this type, a set of separated color data is simultaneously read from the start in the main scanning direction using an image sensor array in which sensor elements to detect red, green and blue are disposed in a stripe. The image reading apparatus using an image sensor array of the stripe type is configured as in FIG. 17. The principle is disclosed in Japanese patent application laid open unexamined under No. 1-237619.
The image sensor array 1400 of the stripe type is constituted as shown in FIG. 2(b). Following this image sensor array 1400, second line reading means 1401 reads red, green and blue data almost simultaneously. And the green scanned position detection means 1700 detects all the scanned positions of the green data on the line.
Then, a red element and a blue element placed on the two sides of each green element read red and blue data respectively. And those red and blue data as well as the green data as a set are mapped on the image memory 110 at the coordinates for the picture element for the green data as if the red and blue data both shared the same picture element position with the green data.
With the image sensor array of the stripe type as in (2), a set of neighboring red, green and blue color data, which are actually read at different positions in the main scanning direction are mapped into the image memory as color data at one and the same position, to input an image.
It has been pointed out that the image reading apparatus of the three-line simultaneous reading type has the following problems:
The time required to obtain three color data at the same position on the document increases with these distancesxe2x80x94the distance between the G sensor line and the R sensor line which shall be given as d-GR picture elements distance and the distance between the G sensor line and the B sensor line as d-GB picture elements distance. To keep with an increase in distance, the read color data delay buffer 1600 would have to be increased in size. Even if the distances between the sensor lines are fixed, the buffer 1600 would have to be larger in size for a larger resolution of the image sensor array.
Another point is that to make it easy for the user to input an image, it is desirable that the scanning direction is not restricted. For the scanning image sensor array is not always moved in one direction but possible to be changed. It is possible that the unit is moved back. In the mapping method as in FIG. 3, the color data to be held in the read color data delay buffer 1600 has to be changed and adapted to the scanning direction in such a case, which makes the image inputting circuit very complicated.
With the stripe reading type as disclosed in unexamined Japanese patent application No. 1-237619, a set of red, green and blue data read in as shown in FIG. 2(b) are originally not at the same picture element position. In the prior art image reading apparatus of the stripe reading type, a plurality of color data originally not at the same position are mapped as those belonging to one and the same position, resulting in color dislocation.
Addressing those problems with the prior art, the present invention is built on such prior art image reading apparatusesxe2x80x94the apparatuses for simultaneously reading color data of a plurality of colors by scanning over the color document with line reading means having a plurality of sensor lines disposed at a specific interval to read color data of a plurality of kinds of colors, usually three colorsxe2x80x94red, green and blue.
As shown in FIGS. 1, 5 and 13, scanned position detection means 106 detects the scanned positions for the respective color data read as described. On the basis of the scanned positions detected by the scanned position detection means 106, coordinates deriving means 10 derives the coordinates to be occupied on the image memory by color data for a plurality of colors read by the line reading meansxe2x80x94coordinates for a picture element for different color data.
Finally, mapping means 109 maps the respective read color data at the coordinates on the image memory obtained by the coordinate deriving means 10.
The coordinates deriving means 10 may be formed of mapping coordinates deriving means 108 and offset size deriving means 107 as shown in FIG. 1.
The scanned position detection means 106 detects the scanned positions for the read color data of a specific kind of color. The scanned positions for the specific color data that can be selected are the top scanned position of the specific color data and the end scanned position. But the kind of color is not restrictive. Color data of some other color at the top and end scanned positions may be used.
On the basis of the specific scanned positions detected by the scanned position detection means 106, the offset size deriving means 107 works out an offset size. The offset size is from the specific scanned position to the corresponding scanned position for the other respective color data that has not been detected. Then, the mapping coordinates deriving means 108 picks up the offset size detected by the offset size deriving means 107. From the offset size, the means 108 derives the scanned positions for all color data read by the aforesaid sensor lines and works out the coordinates to be occupied on the image memory by all the color data.
The coordinates deriving means 10 may also comprise base color coordinates deriving means 500 and missing color coordinates deriving means 501 as shown in FIG. 5.
On the basis of the aforesaid specific scanned position of the sensor lines where the scanned position detection means 106 detected the above color data for the specific color, the base color coordinates deriving means 500 works out coordinates of the picture elements for the aforesaid specific color. That is, the coordinates are to be occupied on the image memory by all the scanned positions of the respective color data on the specific color. From the picture element coordinates for the color data on the specific color found by the base color coordinates deriving means 500, the missing color coordinates deriving means 501 works out the coordinates on the image memory for all color data on the other colors than the aforesaid specific color.
In the examples set forth above, the scanned position for color data for a specific color is detected and the subsequent procedure is performed on the basis of that result. Instead, it is possible to directly process all color data on the respective colors. This method is so configured as shown in FIG. 13. That is, the scanned position detection means 106 detects all the scanned positions for the respective color data and the coordinates deriving means 10 separately derives the coordinates for each color that are to be occupied on the image memory. All the coordinates for the color data read that the line reading means has read are found that way.
The arrangements described above are provided with a plurality of sensor lines, one line for one specific color. The present invention is also applicable to an image reading apparatus of the so-called stripe sensor type in which the sensor elements for the respective colors are lined up on a stripe, that is, one sensor line.
To illustrate, as shown in FIG. 14, the aforesaid color data for a plurality of colors are simultaneously read by scanning the document with second line reading means of the stripe sensor line construction. And the specific color scanned position detection means 1402 detects all the scanned positions for color data for a specific color out of the color data read. On the basis of all the scanned positions thus obtained, the coordinates deriving means 10 derives the coordinates to be occupied on the image memory by all color data that the line reading means has readxe2x80x94the different coordinates for different picture elements corresponding to the color data. And the respective color data read are mapped at the coordinates.
The coordinates deriving means 10 in the aforesaid arrangements may be provided additionally with a sensor-direction offset size deriving means 1403 and second mapping coordinates deriving means 1404.
The sensor-direction offset size deriving means 1403 works out the offset size between the sensor elements to find the scanned position for the other color data that has not been detected. That is done on the basis of the scanned position detected by the specific color scanned position detection means 1402. The second mapping coordinates deriving means 104 works out the picture element coordinates to be occupied on the image memory for all the color data on the basis of the offset size in the sensor direction obtained by the sensor-direction offset size deriving means.
The color data for the coordinates thus obtained can fail to produce image data with a sufficiently high density, making it necessary to interpolate the color data at the coordinates between the mapping coordinates. Needless to say, the interpolation may be effected by either or both of the following methods. One is to work out the color data between the neighboring scanned positions (mapping positions) on the same sensor line. The other involves calculating the color data between the opposed scanned positions (mapping positions) between the neighboring sensor lines for the same color.